These teachings relate generally to spectroscopy, and, more particularly, to sample sensing by terahertz spectroscopy.
Spectroscopy in the terahertz (THz) region is a highly sensitive technique for detecting the gas phase rotational spectrum of a vast number of small compounds that have permanent dipole moments. The terahertz region offers several distinct advantages: i) resolved spectral features for a wide range of species are accessible by the frequency coverage of a single source (currently having 50 GHz to 100 GHz of bandwidth), ii) complete selectivity is possible because of the small intrinsic spectral widths even in “noisy” or “dirty” environments, iii) optimal sensitivity is realized by probing near the peak of the thermal Boltzmann distribution, iv) absolute specificity is achieved since frequencies are traceable to the Rb atomic standard (±2 parts in 1010), and v) absorption signals reflect absolute concentration without need of instrument calibration factors. In particular, rotational spectroscopy is sensitive to molecular structure, and each molecule (even isotopically substituted molecules) has a unique rotational spectrum much like a finger print or bar code. For molecules with only a few heavy atoms, the spectral region at 0.5 THz is near the peak of the Boltzmann distribution at room temperature and therefore, is the most sensitive region for detection of rotational lines. Many of these simple molecules are key atmospheric species (N2O, H2O), volatile organic compounds (formaldehyde, methanol), or indicators of disease states (NO, acetone). For direct absorption studies, the resolution is Doppler limited at room temperature to (1 to 5) MHz at 500 GHz for small molecules because of the thermal velocity distribution. The clear sensitivity advantages together with recent technological advances in sources and receivers make this region well suited for developing an analytical instrument for trace gas analysis.
A number of spectroscopic sensing THz systems using different technologies have demonstrated trace gas detection of different components over the last decade or so. Bigourd et. al. reported a sensor based on photomixing techniques to detect and quantify small quantities of hydrogen cyanide, carbon monoxide, formaldehyde, and water. At present, THz spectroscopy is performed by (i) scanning a high-resolution (1 MHz) narrow-band source with bolometric detection or (ii) time-domain spectroscopy in which a broad (3 THz) spectrum is obtained by measuring the time dependence of the electric field of the THz pulse followed by Fourier transform to the frequency domain. The first method has excellent spectral resolution but is limited in response time, while the second method is fast but has insufficient spectral resolution. Furthermore, these are direct absorption methods that require background correction of the instrument response without sample.
There is a need for terahertz spectroscopy methods that are fast and have excellent spectral resolution. There is also a need for terahertz spectroscopy methods that do not require background correction of the instrument response without sample.